Urethane prepolymers having terminal free isocyanate groups can be cured with moisture contained in the air or a substrate at room temperature, and have been employed for sealants, waterproofing materials, floor covering materials, wall covering materials, paving materials, coatings, adhesives and the like because of their excellent properties such as physical properties of the cured products obtained therefrom and adhesiveness. However, drawbacks to deteriorate their physical properties and appearance by foaming caused by generation of carbon dioxide gas associated with curing have been pointed out.
For the purpose of preventing foaming caused by the generation of carbon dioxide gas when the curing of such urethane prepolymers proceeds, AU-B-20324/88, U.S. Pat. No. 3,743,626, U.S. Pat. No. 5,473,043 and Japanese unexamined patent publication No. 293821/1994 and the like propose to use oxazolidine compounds.
In general, the use of the oxazolidine compounds for preparing urethane prepolymers causes the increase in crosslinking density of the cured products due to the act of the oxazolidine compounds as a crosslinking agent. The modulus of the cured products also increases with the increase in the crosslinking density. Heretofore, the use of such oxazolidine compounds for urethane prepolymers for the sealants has a problem that joint following-up properties of sealants are deteriorated due to the excessively high modulus of the cured products.
Although many sealants usually include thixotropic agents such as fine powder silica and surface-treated calcium carbonate, the hydrophilic fine powder silica, which has much moisture attaching thereon, causes a problem that sealants become viscous during storage. The surface-treated calcium carbonate is problematic in that the surface-treating agent elutes during storage and then coheres to form solid which deteriorates the appearance of the sealants.